1. Field of the Invention
The present invention relates to an apparatus for manufacturing a porous glass preform, and particularly to a structure of a reaction vessel of the apparatus in which glass particles are synthesized through a flame hydrolysis reaction.
2. Description of the Background
The porous glass preform is manufactured by depositing glass particles, as a core, and further, depositing glass particles thereon as a cladding. Typical methods for manufacturing the porous glass preform are an outside vapor deposition method (OVD) and a vapor-phase axial deposition method (VAD).
For example, the following is disclosed in Japanese Patent Laid-open Publication No. Hei. 5-330841:
A glass raw material made of oxygen, hydrogen, and silicon tetrachloride SiCl.sub.4) is fed to an oxyhydrogen burner. The oxyhydrogen burner produces an oxyhydrogen flame, and silicon tetrchloride (SiCl.sub.4) in a glass raw material is subjected to hydrolysis in the oxyhydrogen flame, so that glass particles containing silica (SiO.sub.2) as the main ingredient are produced. The glass particles are deposited, and finally become the porous glass preform.
Such manufacture of the porous glass preform is carried out in a reaction vessel. Japanese Patent Laid-open Publication No. Hei. 5-330841 also discloses this reaction vessel.
The reaction vessels are made of glass or metallic materials.
With the enlargement of the size of the preform in recent years, metallic materials (Fe--Ni, Ni--Cr or other Ni-based alloys, etc.) have come into conventional use. For example, Japanese Utility Model Laid-open Publication No. Hei. 3-74630 discloses a reaction vessel in which a wall is made of acid resistant metallic materials such as Ni or Ni-based alloy, and at least the inner surface thereof is coated with both heat and acid resistant paint.
The porous glass preform must be manufactured under a clean environment, so that impurities do not penetrate into the preform.
Further, the inside of the reaction vessel has a hydrochloric acid atmosphere. Therefore, the reaction vessel is required to have a degree of air tightness to prevent seepage of chlorine or any chlorine compound. In view of the above and also for the purpose of strength, an integral-type reaction vessel which is monolithically formed or which is constructed of several welded parts, has been used. As an optical fiber drawn from one porous glass preform is longer, production efficiency of optical fiber increases. Thus, the porous glass preform has become larger recently. Accordingly, the reaction vessel in which it is produced and contained has also enlarged. Further, during the manufacture of the porous glass preform, the temperature of an oxyhydrogen flame generated from an oxyhydrogen burner is in a range of 2,000 through 2,700.degree. C., the temperature of deposited glass particles is in a range of 800 through 900.degree. C., and the temperature of the reaction vessel wall is in a range of 300 through 400.degree. C. With the enlargement of the reaction vessel the absolute amount of thermal expansion is increased, causing cracks or distortions in the reaction vessel.